In the ceramic art it is often desirable to utilize spray-coating processes to provide decorative, protective, electrically-conductive, reflective or other similarly functional coatings on ceramic articles. A wide variety of solutions, emulsions and suspensions useful for providing such functional coatings are commonly applied to ceramic articles by spraying in liquid form.
In many cases, it is desirable to apply sprayed coating compositions only to selected portions of a ceramic article. The expedient of masking portions of the article which are not to receive the coating composition is then employed because of the difficulty of precisely controlling the spraying process to cover only limited surface areas.
The most important requirement of a good masking material is that it be sufficiently impervious to the sprayed coating composition to prevent contact between the composition and the ceramic surface. In addition, where it is desired that the mask undergo additional processing steps prior or subsequent to the actual spray coating operation, the mask must not interact deleteriously with the ceramic surface or the coating composition during these additional processing steps.
The most common example of additional processing which adversely affects common masking material is high temperature firing. Many ceramic coating operations involve firing at quite high temperatures prior or subsequent to the spray application of the coating composition, and few masking materials will survive such a firing without adversely affecting the coating or the ceramic substrate.
Bulky masking assemblages suitable for mechanical attachment to ceramic articles may be fabricated of refractory materials such as stainless steel or asbestos to provide masks which will withstand a high temperature firing treatment. However, the thickness required to impart reasonable mechanical durability to such an assemblage necessarily produces a thick mask which causes a shadowing or halo effect at the edge portions of a sprayed coating. Thus, thick masks are unacceptable where coating edge thickness is important.
Masking compositions which may be applied to ceramic surfaces to provide thin masking coatings thereon have been employed in the prior art, but many are excessively permeable and most will not withstand high temperature firing without themselves bonding to the ceramic substrate to an objectionable degree. Attempts to circumvent this problem by providing masking compositions composed of refractory materials resistant to bonding and sintering have not been successful because such compositions are commonly porous and thus permeable by sprayed coating compositions.
What is therefore required is a masking composition which is both impermeable to liquid coating compositions and readily removable from ceramic surfaces after exposure to elevated firing temperatures.